Modern fabrication of VLSI circuits makes wide use of deposited films on semiconductor wafers. Deposited films have many uses, such as conducting regions within the device, electrical insulation between metals, and protection from the environment. One of the materials most often deposited is polycrystalline silicon. Among other uses, it may be used as the gate electrode material in MOS devices, as a conducting material for multilevel metallization, and as a contact material for devices with shallow junctions. Deposition gases may be pyrolyzed in a low pressure chemical vapor deposition, LPCVD, reactor to form doped silicon films. U.S. Pat. No. 4,877,753, issued 10/31/89, assigned to Texas Instruments Incorporated and incorporated herein by reference discloses in-situ doping of polysilicon using tertiary butyl phosphine. The copending and coassigned patent application of coinventor Tang filed 12/1/90 entitled "Method and Apparatus for In-Situ Doping of Deposited Silicon", Ser. No. 07/444,900, incorporated herein by reference, discloses using deposition gases of silane SiH.sub.4 and tertiary butyl phosphine, TBP, (C.sub.4 H.sub.11)P in a LPCVD reactor to form doped amorphous films. LPCVD techniques allow for potential advantages in uniform step coverage, precise control of composition and structure, low temperature processing, fast deposition rates, high throughput, and low processing costs. LPCVD exemplary apparatuses include horizontal and vertical reactors.
FIG. 1 illustrates a typical horizontal LPCVD hot wall reactor. The horizontal reactor 10 has a horizontally positioned quartz tube 11. The inside of quartz tube 11 forms the reaction chamber, process chamber, 11a. Three zone furnace 12 heats quartz tube 11. Gas inlet 13a of gas tube 13 introduces deposition gases into one end of reaction chamber 11a. A pump 14 at the other end pumps out the deposition gases. The semiconductor wafers 15 stand vertically in quartz holder (wafer boat, wafer carrier) 16. Each run typically processes 50 to 200 wafers. The wafer boat 16 is loaded into the quartz tube 11 through the load door 17 located at end of quartz tube 11 near gas inlet 13a. A pressure sensor 18 monitors the pressure within reaction chamber 11a. Pressures in the reaction chamber are typically 30 to 250 Pa (0.25 to 2.0 Torr); temperatures range from between about 300 and 900.degree. C.; and gas flows are generally between about 100 and 1000 std. cm.sup. 3 /min.
FIG. 2 depicts an exemplary vertical LPCVD reactor. Thermco Systems, a division of Silicon Valley Group, Inc., manufactures such a vertical reactor. The vertical reactor 20 has a vertically positioned bell shaped quartz tube 21 about forty three inches in length. Vertical reactor 20 is about fifty inches long. The inside of quartz tube 21 forms the reaction chamber, process chamber, 21a. The inside top belled end of quartz tube 21 presents the top of reaction chamber 21a. Resistive heated furnace element 22 provides heat. The portion of vertical reactor 20 heated by 22 is referred as the "heat zone" of the reactor.
In FIG. 2, the deposition gases are introduced into the reaction chamber 21a through two sources: top gas inlet 230a of top gas tube 230; and, bottom gas inlet 231a of bottom gas tube 231. The portion of top gas tube 230 within the heat zone of vertical reactor 20 is short--only about five inches in length--when compared to the portion of bottom gas tube 231 within the heat zone--about 32 inches in length. A pump, not illustrated, connected to the dual flange plenum 24 draws the deposition gases out of the process chamber 21a.
The semiconductor wafers 25 of the vertical reactor of FIG. 2 lie above one another in the horizontal position in the quartz wafer carrier 26. Quartz wafer carrier 26 rests upon quartz pedestal 29. The quartz pedestal 29 and the wafer carrier 26 are loaded into the process chamber 21a through the load door 27 at the bottom of vertical LPCVD reactor 20. A pressure sensor, not illustrated, connected to pressure sensor tube 28 monitors the pressure and a temperature sensor, not illustrated, attaches to temperature sensor tube 28a to monitor temperature. The pressures, temperatures, and gas flows for vertical LPCVD reactor 20 are similar to those of horizontal LPCVD reactor 10 of FIG. 1.
FIG. 3 illustrates a top view of the outside top belled end of quartz tube 21 of FIG. 2 and shows in particular the routing of top gas tube 230.
A problem exist when using vertical LPCVD reactors to form suitable in-situ doped silicon films such as polysilicon and amorphous silicon on wafers for VLSI submicron circuit devices, such as 16 mega bit dynamic random access memories "16 MB DRAMs". The small geometries of these devices require stringent deposition tolerances throughout the wafers within a lot run in order to have sufficient yield and throughput. The dopant gases passing through the top gas tube do not have sufficient time to heat before entering into the top of the process chamber. These dopant gases enter the top of the reaction chamber essentially unheated and cold; they do not become fully heated and do not reach their decomposition temperature until they reach the lower portion of the reaction chamber; they do not fully decompose until their decomposition temperature is reached; therefore, they do not fully decompose at the upper portion of the process chamber. The dopant gases entering into the reaction chamber 21a through the top of quartz tube 21 through top gas inlet 230a of top gas tube 230 therefore are not as thoroughly incorporated into the formation of in-situ deposited silicon films on the wafers positioned in wafer boat 26 near the upper portion of process chamber 21a. This leads to silicon film thickness problems in the wafers and doping uniformity problems in the wafers causing the wafers in the upper portion of wafer boat 26 to be unsuitable for submicron VLSI 16 MB DRAM processing.
One potential solution to this problem is to simply not run as many wafers in a lot. The reduced number of wafers can all be positioned lower in the wafer carrier 26 so that they reside lower in the reaction chamber 21a, thereby making it more likely that the dopant gases entering the process chamber from the top will be fully decomposed when they reach the wafers. This solution is undesirable, however, because it reduces lot throughput.
It is an object therefore of this invention to provide an improved vertical LPCVD reactor.
Other objects and benefits of this invention will be apparent to those of ordinary skill in the art having the benefit of the description to follow herein.